1. Field of the Invention
The field of the invention is fluidized catalytic cracking (FCC) of heavy hydrocarbon feeds and selective catalytic reduction (SCR) of nitrogen oxides from a FCC regenerator.
2. Description of Related Art
Catalytic cracking is the backbone of many refineries. It converts heavy feeds into lighter products by catalytically cracking large petroleum molecules into smaller molecules. Catalytic cracking operates at low pressures, without hydrogen addition, in contrast to hydrocracking, which operates at high hydrogen partial pressures.
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hydrocarbon feed vaporizes and the hot catalyst cracks the feed at 425xc2x0 C.-600xc2x0 C., usually 460.xc2x0 C.-560.xc2x0 C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, approximately, 500xc2x0 C.-900xc2x0 C., usually 600xc2x0 C.-750xc2x0 C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for conversion of carbon monoxide in the regenerator for a full burn unit or in a CO boiler for a partial burn unit. In a full burn unit the flue gas temperature is normally reduced by a heat recovery system. After the heat recovery system or the CO boiler, the flue gas is normally discharged into the atmosphere or treated with an air pollution control system and then discharged to the atmosphere.
Catalytic cracking is endothermic. The heat for cracking is supplied at first by the hot regenerated catalyst from the regenerator. Ultimately, it is the feed which supplies the heat needed to crack the feed. Some of the feed deposits as coke on the catalyst, and the burning of this coke generates heat in the regenerator, which is recycled to the reactor in the form of hot catalyst.
Catalytic cracking has undergone progressive development since the 1940s. Modern fluid catalytic cracking (FCC) units use zeolite catalysts. Zeolite-containing catalysts work best when coke on the catalyst after regeneration is less than 0.1 wt %, and preferably less than 0.05 wt %.
To regenerate FCC catalyst to this low residual carbon level and to burn CO completely to CO2 within the regenerator (to conserve heat and reduce air pollution) many FCC operators add a CO combustion promoter. U.S. Pat. Nos. 4,072,600 and 4,093,535, incorporated by reference, teach use of combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
Catalyst regeneration usually causes formation of NOx, either in the regenerator, if operating in full CO combustion mode or in a downstream CO boiler, if operating in partial CO combustion mode. NOx emissions are becoming more of a problem, as FCC units are being forced to process lower quality feeds containing more NOx precursors, and as environmental regulations become more strict.
There are many approaches towards operating the FCC unit to reduce NOx emission, various catalyst additives, segregated cracking of different feeds, and regenerator modifications. These are all helpful, but can only achieve a modest reduction in NOx emissions. Some refiners have to do more or increasingly anticipate the need to do more, and now resort to flue gas treatments to remove NOx. There are commercially proven two primary NOx flue gas treatments commercially available, thermal and catalytic.
Thermal destruction of nitrogen oxides (DENOx) involves operation at 870xc2x0 C.-980xc2x0 C. with urea or ammonia addition to reduce NOx. Capital costs can be moderately high, because of the high temperatures, and operating costs can be higher than desired, again because a large volume gas stream must be heated in the case of a full CO combustion FCC. Thermal DENOx is preferred by many refiners for FCC use because it works with no catalyst. A drawback to this approach is that the maximum amount of NOx reduction achievable is typically about 50%. This is often denoted as the SNCR process. Catalytic reduction of NOx, the SCR process, is a proven technology used to reduce NOx emission for many refinery processes. It operates at moderate temperatures, well below those of FCC regenerators, so operating costs are moderate. It adds a roughly stoichiometric amount of ammonia to a NOx containing flue gas stream and relies on a catalyst, usually honeycomb monoliths, to promote the reduction of NOx by NH3. The process works well with flue gas from furnaces, which can have moderate amounts of NOx and other gaseous pollutants, but are relatively free of particulates.
However, because the catalyst must be protected from fouling by catalyst particles, SCR units need to be located downstream of the third stage separator or even sometimes after an ESP that will reduce the particle loading below 50 mg/Nm3. The ESP may be used to protect the SCR and/or may also be used in order to reduce the particulate emissions into the atmosphere as a requirement in the environmental permit for an oil refinery. In either case, an ESP lacks the capability to operate reliably over the length of time required by a lot of oil refiners for best economical practice. Typically, refiners operate the FCC 3 to 5 years between turnarounds without opportunity for an ESP outage. Third Stage Separators (TSSs) have often been used to protect FCC turbo-expanders from catalyst abrasion and in one case was used to protect an SCR. TSSs have a relatively low cost and are reliable; however, the drawback of TSSs for the case of stack emissions is that the TSS underflow (usually about 3% of the total gas flow) must be further treated or, if not further treated, the flue gas emitted at the stack will have the same catalyst load as before treatment with the TSS.
The equipment used for treatment of TSS underflow would normally be a fourth stage cyclone or a ceramic filter designed for high temperature. A fourth stage cyclone typically only removes about 75% of the catalyst particles from the TSS underflow resulting in an ultimate catalyst emission of  greater than 25% of the original catalyst emission before TSS treatment. This is too much to be eventually discharged to the environment. The ceramic filter is efficient at removing catalyst at essentially 100% but the cost and reliability for continuous operation make it less attractive in many cases. The ceramic filter has only been used on a small number of FCC TSS applications. Another problem associated with the filter is that it is a piece of equipment which is more prone to shutdowns, and corrosion problems have been associated with shutdowns, because sulfuric acid will condense when temperature is lowered.
In today""s modern refineries, stack emission concerns for the FCC are mainly particulate (catalyst), SOx, and NOx. NOx is a more recent concern worldwide (as for Japan in the past) and is now the target of a large number of environmental regulators. SOx emissions can be removed in several ways including:
1) SOx reduction additive can be added to the FCC catalyst.
2) Pretreating the feed by hydrocracking or hydrotreating before introduction of oil to the FCC.
3) Operating on lower sulfur crudes.
4) Wet scrubbing with an alkali after the FCC to absorb SOx from the gas. The drawbacks of 1) to 3) above are:
1) SOx reduction catalystxe2x80x94can be expensive and often cannot meet recent permit requirements.
2) Hydrocracking/Hydrotreatingxe2x80x94generally very expensive and usually only economical only if final product sulfur requirements dictate this treatment and not the FCC stack emissions.
3) Lower sulfur crudes xe2x80x94reduces refinery margins so it is deemed uneconomical.
Since wet scrubbers can be used to remove both particulate and SOx concurrently, one may propose that a typical process for removal of all three pollutants would consist of a wet scrubber for SOx and particulate control followed by an SCR for NOx control. Optimum temperature for SCR, however, is much higher than the adiabatic quench temperature of a wet scrubber. Therefore, in order to utilize a wet scrubber and SCR in the order proposed, a reheat system would be required with additional capital/operating costs, significant real estate requirements, and questionable long-term reliability.
The present invention provides a fluidized catalytic cracking flue gas cleaning process wherein the flue gas from the said process is passed to a third stage cyclone separator; wherein the overflow of the third stage separator is connected to an SCR catalytic unit; wherein the underflow of the third stage separator containing both the recovered solids and the gas is remixed to the outlet of the SCR; wherein the resulting gas mixture is fed to a wet scrubber that will remove more than 90% of the sulfur dioxide and more than 80% of all solid particles greater than 3 microns.
As a variant, the underflow of the third stage is fed to a fourth stage and the overflow of the fourth stage separator is remixed with the outlet of the SCR; wherein the resulting gas mixture is fed to a wet scrubber that will remove more than 90 of the sulfur dioxide and more than 80% of all solid particles greater than 3 microns. With such an embodiment, since the wet scrubber will remove the fine dust, there is no need to install a gas solid separator like a ceramic filter on the underflow of the third stage separator, thus providing a substantial economy.